This invention relates to plasma channels, to display devices comprising plasma channels, and to plasma-addressed liquid crystal display panels commonly referred to as "PALC" display devices using such channels. PALC devices comprise, typically, a sandwich of: a first substrate having deposited on it parallel transparent column electrodes, commonly referred to as "ITO" columns or electrodes since indium-tin oxides are typically used, on which is deposited a color filter layer; a second substrate comprising parallel sealed plasma channels corresponding to rows of the display crossing all of the ITO columns and each of which is filled with a low pressure ionizable gas, such as helium, neon and/or argon, and containing spaced cathode and anode electrodes along the channel for ionizing the gas to create a plasma, which channels are closed off by a thin transparent dielectric sheet; and a liquid crystal (LC) material located between the substrates. The structure behaves like an active matrix liquid crystal display in which the thin film transistor switches at each pixel are replaced by a plasma channel acting as a row switch and capable of selectively addressing a row of LC pixel elements. In operation, successive lines of data signals representing an image to be displayed are sampled at column positions and the sampled data voltages are respectively applied to the ITO columns. All but one of the row plasma channels are in the de-ionized or non-conducting state. The plasma of the one ionized selected channel is conducting and, in effect, establishes a reference potential on the adjacent side of a row of pixels of the LC layer, causing each LC pixel to charge up to the applied column potential of the data signal. The ionized channel is turned off, isolating the LC pixel charge and storing the data voltage for a frame period. When the next row of data appears on the ITO columns, only the succeeding plasma channel row is ionized to store the data voltages in the succeeding row of LC pixels, and so on. As is well known, the attenuation of the backlight or incident light to each LC pixel is a function of the stored voltage across the pixel. A more detailed description is unnecessary because the construction, fabrication, and operation of such PALC devices have been described in detail in the following U.S. patents and publication, the contents of which are hereby incorporated by reference: 4,896,149; 5,077,553; 5,272,472; 5,276,384; and Buzak et al., "A 16-Inch Full Color Plasma Addressed Liquid Crystal Display", Digest of Tech. Papers, 1993 SID Int. Symp., Soc. for Info. Displ. pp. 883-886.
A partial perspective view of the PALC display described in the 1993 SID Digest is shown in FIG. 2. The method described in the referenced publication for making the plasma channels is to chemically etch a flat glass substrate to form parallel semi-cylindrically shaped recesses defined by spaced ridges or mesas and to bond on top of the mesas a thin dielectric cover sheet having a thickness in the range of about 30-50 .mu.m.
The above construction and its fabrication encounters certain problems. Since the channel electrodes must be patterned on the sloping sidewall of the channel, the dimensions and placement of the electrodes cannot be accurately controlled. Moreover, since slight variations in processing conditions can alter the etch rate, the channel etching process is difficult to control; hence the depth of the channel, which is dependent on control of the etching process, is difficult to control.
European Patent 0 500 084 A2 describes the formation of channels by patterning of electrodes on a flat substrate, providing spacers on the flat substrate, and placing the thin glass sheet on top of the spacers. The discharge space thus extends continuously across the electrodes. However, the continuous discharge space will lead between channels to crosstalk which is difficult to avoid. Moreover, the spacers have to be formed on the flat substrate by deposition and/or etching processes, such as screen printing. Since the spacers have to be as thick as the required channel depth (.about.100 microns or more) the fabrication of the spacers adds complexity to the process.
European Patents 0 500 085 A2 and 0554 851 A1 describe the formation of channels by screen printing partition walls. However, this is also a difficult process, which may require multiple coats to obtain the required wall height.